I haven’t messed with the Rhythm channels on the units, so I’m only going by the Lead modes. So on Channel 1, I noticed when tweaking the Lead Drive, there was a lot of crackling. As I started playing, the volume dropped, and now when you turn the gain on 10 it’s like it’s on 2. When you turn the drive down to 3, its at full blast.

I noticed on a lot of the knobs (Lead drive, Volume, Master, Lead Master) that it has a major jump from 0-1 or 2, then from there it goes up to 3 or 4 then every adjustment past that has no effect. The same with channel 2, but it’s not as drastic.

I bought some contact cleaner to maybe clean the pots up a bit and it helped the crackling and it made the Lead Drive on Channel 1 not have a volume drop at 4. It peaks around 2, then is slightly less driven from 3-10. And the settings on 3 through 10 are all the same. If you turn it to 1, major cut, then 0 is obviously nothing.

The real kicker is that on Channel 2. NONE of the Pull push knobs work. Pulling the knob has no effect on the sound, and that’s wrong.

I obviously need this to be serviced. I’ve checked the tubes and everything “seems” to be okay. I could be very well mistaken. Could I just be missing a bad tube? Dirty pots? Bad pots? Maybe a Capictior?

Edit: I have a picture of the inside of the Preamp. I can post a link to iMGr since on a mobile device and it can’t upload pictures. I can take more or focus on certain parts when I get home.

I had very similar problems with my quad after a few years of little usage. I brought it to a Mesa authorized service place. My issues were a volume drop on R1 after playing for 15+ minutes and L2 would cut out and randomly drop volume.

The tech had it for about a week and when I went to pick it up, he said it needed a really good cleaning. This was about 3 years ago and it's worked perfect since.

I do notice that L2 lead master seems to go full volume (or at least the volume where it starts to hurt my ears) at around 2.5. 2.0 is too low, and 3.0 gets me yelled at. Lol. I've been compensating by using the output.

As for your push/pull issue, I never had that problem with the quad but I did have the same issue with my first Mark IV. Brought that in and the tech pulled the chassis, removed the knobs and pots, tested and cleaned. Haven't had an issue since.

Give a call to Mesa. They're real good about trouble shooting over the phone/email.

First thing: Those pots are probably pretty dirty, that explains the crackling and some of the sudden erratic levels. Contact cleaner is a good first bet. And that seemed to handle a couple of those issues.

Second, there's components called LDRs inside these amps (Light Dependent Resistors) which are used for the channel and mode switching. They have a lifetime of around 20-40 years, and since they were all built around 88 give or take a couple years they're all 30 years old now and likely to be failing. If they fail then the switching between L1 and L2 might not be functioning. When the Push/pull pots don't work on L2, try toggling the Push/Pull on L1 and see if those are affecting it instead. (Also note that PULL DEEP is not channel specific. The Pull DEEP on channel 1 affects the output A and the PULL DEEP on channel 2 affects output B, so those ones are intended to behave differently).

Some ailing filter caps could also be causing issues, they also have a lifetime that's likely up by now.

A tech will have the cools to test these things and at least give you some answers.